OK. So this keeps getting me. Thus I'm blogging the answer as an aide memoire to myself.
If you have an ASP.NET site in IIS (typically one you've just installed or updated) and all you get is a blank page in internet explorer (or a nasty xml error in firefox)..
If you also use Global.asax to override the error handling, then you will need to temporarily uncomment that - basically something fundamental has gone wrong, and not even the error handler is working.
Normally this means the site can't load its configuration file(s), can't get to it's database, etc. Best thing is to uncomment the handling in global.asax and try again - then put it back once it's fixed!